Primary biliary cirrhosis (PBC) is a well-characterized autoimmune liver disease that results in the destruction of intrahepatic bile ducts with progressive inflammatory scarring (Kaplan M. M., Adv Intern Med 32:359-377, 1987). The disease is marked by an autoantibody response to mitochondria, originally identified using immunofluorescence (Berg, P. A. et al., 20 Hepatology 2:123-131, 1986; Frazer, I. H. et al., J. Immunol 135:1739-1745, 1985; Kenna, J. G. et al., J. Immunol. Methods 73:401-413, 1984; Kaplan, M. M. et al, Hepatology 4: 727-730, 1984; Walker, J. G. et al., Lancet 1:827-831, 1965).
A characteristic serologic feature observed in sera from patients with PBC is the presence of high titers of antibodies directed against mitochondrial antigens (AMA) (Mackay I. R., N Engl J Med 1958; 258:185-187; Walker, J. G. et al., Lancet 1965; 1:827-831; Berg, P. A. et al., J Exp Med 1967; 126:277-290; Gershwin M E and Mackay I R. Gastroenterology 1991; 100: 822-833). The major autoantigens recognized by sera from PBC patients have been identified as members of the 2-oxo-acid dehydrogenase complex (2-OADC) family, including the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2), the E2 subunit of the branched chain 2-oxo-acid dehydrogenase complex (BCOADC-E2), the E2 subunit of the 2-oxo glutarate dehydrogenase complex (OGDC-E2), E1" subunits of PDC and protein X (Yeaman, S. J. et al., Lancet 1988; 1:1067-1070; Van de Water J, et al., J Exp Med 1988; 167:1791-1799; Fussey S P M, et al., Proc Natl Acad Sci USA 1988;85:8654-8658; Fregeau D R, et al., J Immunol 1989;142;3815-3820; Surh C D, et al., Hepatology 1989;10:127-133; Fregeau D R, et al., J Immunol 1990;144:1671-1676; Fregeau D R, et al., Hepatology 1990; 1 1:975-981).
Among these enzyme components, the E2 component of PDC, or dihydrolipoamide dehydrogenase, is the major autoantigen of PBC since the serum samples of the majority of patients (80 to 90%) contain PDC-E2 specific AMA. In addition to PDC-E2, approximately 60% of patients with PBC are also reactive with BCOADC-E2 (Mutimer D J, et al., Hepatology 1989;10:403-407; Leung P S C, et al., Hepatology 1992;15:367-372). Interestingly, 4% to 13% of sera from patients with PBC recognize only the BCOADC-E2 but not the PDC-E2 (Leung P S C, et al., Hepatology 1992;15:367-372; Van de Water J, et al., N Engl J Med 1989;320:1377-1380; Iwayama T, et al., Int Arch Allergy Immunol 1992;99:28-33). The E2 component of OGDC-E2, dihydrolipoamide succinyltransferase, is recognized in 30% to 80% of sera from patients with PBC (Fregeau D R, et al., Hepatology 1990; 1 1:975-981; Mutimer D J, et al., Hepatology 1989;10:403-407; Leung P S C, et al., Inflammatory hepatobiliary cirrhosis. pp. 1429-1443. 1996. In: Clin. Immunology. Principles and Practice. Rich. R. R.(Ed). Mosby. Year book. Inc. St. Louis, Mo., USA). The immunodominant epitopes of PDC-E2 and BCOADC-E2 have been previously mapped to their lipoic acid binding domains (Surh C D, et al., J Immunol 1990;144:3367-3374; Leung P S C, et al., Hepatology 1995; 22:505-513). Although PDC-E2 contains two lipoic acid binding domains, the reactivity of AMA is about 100 times stronger to the inner lipoyl domain (Van de Water J, et al., J Exp Med 1988; 167:1791-1799; Surh C D, et al., Hepatology 1989;10:127-133).
The complementary DNA (cDNA) of PDC-E2, BCOADC-E2 and PDC-E1" have been isolated and used to produce recombinant proteins designated PDC-E2, BCOADC-E2, and PDC-E1", respectively. A strong reactivity of patient AMA to these proteins by immunoblotting and enzyme-linked immunosorbent assays (ELISA) was demonstrated (Leung P S C, et al., Hepatology 1992;15:367-372; Van de Water J, et al., N Engl J Med 1989;320:1377-1380; Gershwin M E, et al., J Immunol 1987;138:3525-353 1; Coppel R L, et al., Proc Natl Acad Sci USA 1988; 85:7317-7321; Griffin T A, et al., J Biol Chem 1988;263:14008-14014; Danner DJ, et al., J Biol Chem 1989;264:7742-7746; Ho L, et al., Proc Natl Acad Sci USA 1989;86:5330-5334; Surh C D, et al., Hepatology 1989;9:63-68; Iwayama T, et al., J Autoimmunity 1991 ;4:769-778).